Bulk transition metal oxides that exhibit ferroelectric, piezoelectric, converse piezoelectric, pyroelectric, magnetoresistive, and high-permittivity dielectric properties have been widely used in industry to fabricate various memory devices, ferroelectric capacitors, electromechanical actuators, resonators, sensors, optical switches and waveguides. For example, these transition metal oxides may be used in non-volatile ferroelectric random-access memory (NVFRAM) devices. The basis of NVRAM devices may be the ferroelectric property of the material. Ferroelectric properties of a material include the spontaneous permanent dipole moment exhibited by the material that can be reoriented by external electric field. NVFRAM devices use non-volatile ferroelectric polarization in lieu of field-effect gates and modulate the conductance of the doped semiconductor materials. Nonvolatile FRAMs may be used in consumer electronics, such as smart cards, and may be used as the next-generation memory architecture to replace dynamic RAMs (DRAMs).
These metal oxides may also be used as ferroelectric dynamic random-access memory (FDRAM) devices. Ferroelectric materials exhibit a high permittivity, for example, ∈>300 for barium strontium titanate compared to ∈=7 in silicon oxide, which may be exploited to make charge-storage and DRAM devices. FDRAMs work in a similar fashion to conventional DRAMs and store information as charge in a capacitor. The high permittivity of a ferroelectric material allows the significant reduction in the capacitor size and hence the size of the whole RAM device.
Ferroelectric oxides typically exhibit a host of other related properties, such as piezoelectricity, pyroelectricity, and large nonlinear optical coefficients. Central to all these diverse properties of ferroelectric oxides is the structural phase transition of the underlying oxide lattice, wherein below a certain phase transition temperature, the crystal lattice as a whole develops a spontaneous dipole moment or polarization and becomes ferroelectric. The same distortion of the unit cell, added together coherently throughout the crystal, also results in the deformation of the whole crystal that leads to piezoelectricity. In addition, owing to the loss of the inversion symmetry, the crystal in the tetragonal phase exhibits a large second order optical susceptibility that is responsible for second harmonic generation.
Converse-piezoelectric, that is, the deformation of the material upon the application of the electric field, and piezoelectric applications of bulk transition metal oxides may also be used as the basis of bulk and micrometer-sized electromechanical actuators, pumps, and more generally the whole class of micro-electromechanical systems (MEMS). Examples of converse-piezoelectric applications include piezoelectric actuators employed to move and position an object down to Ångstrom precision and the piezoelectric fluid pumps used in inkjet-printer heads. The piezoelectric property exhibited by the material, i.e., the development of voltage (or surface charge) upon the deformation of materials, is the physical basis of force and motion sensors, and resonators. Some examples of sensor applications are piezo-cantilevers used in atomic force microscopy to sense feature heights and accelerometers used to deploy air bags in motor vehicles. The resonator applications utilize both converse-piezoelectric and piezoelectric properties of the material to drive mechanical oscillations of the material using electrical inputs and to detect these resonant oscillations electrically. These resonators can be used as high-frequency bandpass filters in telecommunication systems, replacing bulky inductive-capacitance (L-C filters.
The pyroelectric properties exhibited by bulk transition metal oxide materials, including the change of voltage between opposite faces of the material with a change in temperature, is the physical basis of sensitive temperature and infrared sensors. Dielectric properties of bulk transition metal oxides may lend themselves to use in integrated circuits and other semiconductor applications.
Another interesting member of the transition metal oxide family are the doped lanthanum manganites. In the bulk, these transition metal oxides have stimulated considerable scientific and technological interest due to its amazing variety of electronic and magnetic properties, including charge and orbital ordering, metal/insulator and ferromagnet/antiferromagnet transitions, lattice and magnetic polarons, and colossal magnetoresistance (CMR).
Magnetoresistive perovskite manganites are currently used in many business sectors such as consumer electronics, the wireless telephone industry, and the automobile industry. These industries currently employ large and expensive magnetic field sensors in their products. The development of nanocrystalline manganite sensors could greatly impact these fields.
Experimental studies have been performed on the effects of reduced dimensionality on the phase transitions of metal oxides, including thin film ferroelectric oxides and single crystal samples. However, existing preparation of nanocrystal solids of ferroelectric oxides for example, such as sol-gel synthesis and co-precipitation have yielded highly agglomerated samples with poor crystalline quality. No general synthetic route has existed for the synthesis of nanocrystals with more than two elements.
Previous investigations of thin-film and nanocrystalline samples have shown that their physical properties are critically dependent on their dimension. Despite intensive experimental efforts, however, a general method to synthesize well-isolated crystalline nanostructures of for example, perovskite oxides has been lacking.